A ferrous-alloy article on which a vanadium carbide layer or niobium carbide layer is formed has improved corrosion resistance against molten metals and also has improved wear resistance. A niobium carbide layer has better corrosion resistance and better appearance than those of a vanadium carbide layer. However, niobium carbide has some defects. For example, when low-alloy steel is used as base or mother material, a carbide layer (formed with niobium carbide) has a tendency to crack or peel off. It has been found that a ferrous alloy article with a surface on which a layer of mixed carbide of Va-Group elements is formed has imparted to it such features that the article has the merits of both carbide layers of the individual Va-Group elements.
To form a mixed carbide layer on the surface of a ferrous-alloy article, immersing and keeping the ferrous alloy in a molten borate bath, to which powder of the pure metals or alloys of vanadium and niobium are added, has been proposed (Japanese Pat. No. 669,246). This method has remarkable utility, but also has the following defects:
(1) The metal powder (added to the molten borate bath) tends to build up and thus collect at the bottom of the vessel. When an article being treated is immersed in or contacted with the collected metal powder, some metal powder adheres to and thus impairs the smoothness of the surface of the article. Therefore great care must be taken while dipping the article into the bath.
(2) The effective volume in which articles to be treated are introduced is limited and productivity is reduced to a low level due to the first noted defect.
(3) Since collected or built-up metal powder tends to be sintered (particles adhere to each other) in a grain, the total surface area of the powder becomes smaller and the dissolution of carbide-forming elements in borax or borate is reduced. In order to overcome this, particles collected at the bottom part of the vessel must be stirred up from time to time. This necessitates considerable work.
(4) When metal powder contacts the surface of an article being treated, the powder adheres to and mars the surface condition of the article.
(5) A necessarily large amount of metal powders added to a salt bath increases the viscosity of the bath. The amount of treating material which adheres to an article treated in such bath and is taken out of the bath with the treated article increases with increased viscosity.
(6) The increase in viscosity of the salt bath decreases fluid character and reduces convection within the bath; this results in reducing the uniformity in temperature distribution throughout the bath. The effective volume of the bath-containing vessel is thus further limited.